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  <div class="section" id="to3-automated-python-2-to-3-code-translation">
<span id="to3-reference"></span><h1>26.4. 2to3 - Automated Python 2 to 3 code translation<a class="headerlink" href="#to3-automated-python-2-to-3-code-translation" title="Permalink to this headline">¶</a></h1>
<p>2to3 is a Python program that reads Python 2.x source code and applies a series
of <em>fixers</em> to transform it into valid Python 3.x code.  The standard library
contains a rich set of fixers that will handle almost all code.  2to3 supporting
library <tt class="xref docutils literal"><span class="pre">lib2to3</span></tt> is, however, a flexible and generic library, so it is
possible to write your own fixers for 2to3.  <tt class="xref docutils literal"><span class="pre">lib2to3</span></tt> could also be
adapted to custom applications in which Python code needs to be edited
automatically.</p>
<div class="section" id="using-2to3">
<h2>26.4.1. Using 2to3<a class="headerlink" href="#using-2to3" title="Permalink to this headline">¶</a></h2>
<p>2to3 will usually be installed with the Python interpreter as a script.  It is
also located in the <tt class="docutils literal"><span class="pre">Tools/scripts</span></tt> directory of the Python root.</p>
<p>2to3&#8217;s basic arguments are a list of files or directories to transform.  The
directories are to recursively traversed for Python sources.</p>
<p>Here is a sample Python 2.x source file, <tt class="docutils literal"><span class="pre">example.py</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">greet</span><span class="p">(</span><span class="n">name</span><span class="p">):</span>
    <span class="k">print</span> <span class="s">&quot;Hello, {0}!&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
<span class="k">print</span> <span class="s">&quot;What&#39;s your name?&quot;</span>
<span class="n">name</span> <span class="o">=</span> <span class="nb">raw_input</span><span class="p">()</span>
<span class="n">greet</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
</pre></div>
</div>
<p>It can be converted to Python 3.x code via 2to3 on the command line:</p>
<div class="highlight-python"><pre>$ 2to3 example.py</pre>
</div>
<p>A diff against the original source file is printed.  2to3 can also write the
needed modifications right back to the source file.  (A backup of the original
file is made unless <em class="xref">-n</em> is also given.)  Writing the changes back is
enabled with the <em class="xref">-w</em> flag:</p>
<div class="highlight-python"><pre>$ 2to3 -w example.py</pre>
</div>
<p>After transformation, <tt class="docutils literal"><span class="pre">example.py</span></tt> looks like this:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">greet</span><span class="p">(</span><span class="n">name</span><span class="p">):</span>
    <span class="k">print</span><span class="p">(</span><span class="s">&quot;Hello, {0}!&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">name</span><span class="p">))</span>
<span class="k">print</span><span class="p">(</span><span class="s">&quot;What&#39;s your name?&quot;</span><span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="nb">input</span><span class="p">()</span>
<span class="n">greet</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
</pre></div>
</div>
<p>Comments and exact indentation are preserved throughout the translation process.</p>
<p>By default, 2to3 runs a set of predefined fixers.  The <em class="xref">-l</em> flag lists
all available fixers.  An explicit set of fixers to run can be given with
<em class="xref">-f</em>.  Likewise the <a class="reference external" href="../using/cmdline.html#cmdoption-x"><em class="xref">-x</em></a> explicitly disables a fixer.  The
following example runs only the <tt class="docutils literal"><span class="pre">imports</span></tt> and <tt class="docutils literal"><span class="pre">has_key</span></tt> fixers:</p>
<div class="highlight-python"><pre>$ 2to3 -f imports -f has_key example.py</pre>
</div>
<p>This command runs every fixer except the <tt class="docutils literal"><span class="pre">apply</span></tt> fixer:</p>
<div class="highlight-python"><pre>$ 2to3 -x apply example.py</pre>
</div>
<p>Some fixers are <em>explicit</em>, meaning they aren&#8217;t run by default and must be
listed on the command line to be run.  Here, in addition to the default fixers,
the <tt class="docutils literal"><span class="pre">idioms</span></tt> fixer is run:</p>
<div class="highlight-python"><pre>$ 2to3 -f all -f idioms example.py</pre>
</div>
<p>Notice how passing <tt class="docutils literal"><span class="pre">all</span></tt> enables all default fixers.</p>
<p>Sometimes 2to3 will find a place in your source code that needs to be changed,
but 2to3 cannot fix automatically.  In this case, 2to3 will print a warning
beneath the diff for a file.  You should address the warning in order to have
compliant 3.x code.</p>
<p>2to3 can also refactor doctests.  To enable this mode, use the <a class="reference external" href="../using/cmdline.html#cmdoption-d"><em class="xref">-d</em></a>
flag.  Note that <em>only</em> doctests will be refactored.  This also doesn&#8217;t require
the module to be valid Python.  For example, doctest like examples in a reST
document could also be refactored with this option.</p>
<p>The <a class="reference external" href="../using/cmdline.html#cmdoption-v"><em class="xref">-v</em></a> option enables output of more information on the translation
process.</p>
<p>When the <em class="xref">-p</em> is passed, 2to3 treats <tt class="docutils literal"><span class="pre">print</span></tt> as a function instead of
a statement.  This is useful when <tt class="docutils literal"><span class="pre">from</span> <span class="pre">__future__</span> <span class="pre">import</span> <span class="pre">print_function</span></tt> is
being used.  If this option is not given, the print fixer will surround print
calls in an extra set of parentheses because it cannot differentiate between the
print statement with parentheses (such as <tt class="docutils literal"><span class="pre">print</span> <span class="pre">(&quot;a&quot;</span> <span class="pre">+</span> <span class="pre">&quot;b&quot;</span> <span class="pre">+</span> <span class="pre">&quot;c&quot;)</span></tt>) and a
true function call.</p>
</div>
<div class="section" id="module-lib2to3">
<h2>26.4.2. <tt class="xref docutils literal"><span class="pre">lib2to3</span></tt> - 2to3&#8217;s library<a class="headerlink" href="#module-lib2to3" title="Permalink to this headline">¶</a></h2>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">The <tt class="xref docutils literal"><span class="pre">lib2to3</span></tt> API should be considered unstable and may change
drastically in the future.</p>
</div>
</div>
</div>


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<li><a class="reference external" href="">26.4. 2to3 - Automated Python 2 to 3 code translation</a><ul>
<li><a class="reference external" href="#using-2to3">26.4.1. Using 2to3</a></li>
<li><a class="reference external" href="#module-lib2to3">26.4.2. <tt class="docutils literal"><span class="pre">lib2to3</span></tt> - 2to3&#8217;s library</a></li>
</ul>
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